Conventionally, as an ultrasonic actuator including an actuator having a piezoelectric element, an ultrasonic actuator disclosed in Patent Document 1 has been known.
The ultrasonic actuator of Patent Document 1 includes an actuator body formed of a piezoelectric element and a driver element fixed onto the actuator body.
The actuator body is formed of a flat plate-shaped piezoelectric element having a longitudinal direction. In the actuator body, longitudinal vibration (i.e., so-called expanding/contracting vibration) along a longitudinal direction of the piezoelectric element and bending vibration along a lateral direction of the piezoelectric element are induced in a cooperated manner by respectively applying alternating currents having different phases to two electrode pairs each of which includes two electrodes diagonally arranged. As a result, the driver element makes an orbit motion, more specifically, an elliptical motion in a plane of the piezoelectric element including a longitudinal direction and a lateral direction.
Patent Document 2 discloses an ultrasonic motor (ultrasonic actuator). Patent Document 2 discloses a technique in which in an ultrasonic motor, a driving frequency is set to a higher value than a resonance frequency of the ultrasonic motor and the frequency is reduced from the higher value at a start of driving the ultrasonic motor.
Furthermore, Patent Document 3 discloses a control system for an ultrasonic motor (ultrasonic actuator). Patent Document 3 discloses a technique in which a driving frequency at which a drive current for the ultrasonic motor is minimum is searched and the driving frequency is fixed to the obtained frequency, thereby performing control.
Patent Document 1: Japanese Published Patent Application No. 2004-304963
Patent Document 2: Japanese Published Patent Application No. H7-264882
Patent Document 3: Japanese Published Patent Application No. 2006-115583